Silica (SiO2) coating on metal nanoparticles (NPs) and metal oxide NPs is a promising approach for many value-added applications, such as glass coatings, paints, water and air purification, antibacterials, diagnostics, sensors and cosmetics. SiO2-coated metal NPs and metal oxide NPs have unique features of photocatalytic activity that enables self-cleaning of the surface, as well as water and air treatment, due to the ability of the SiO2-coated metal NPs and metal oxide NPs to remove low levels of organic pollutants. The value-added SiO2-coated metal NPs and metal oxide NPs take advantage of the coated SiO2 layer as it greatly enhances the dispersibility and stability of the metal NPs and metal oxide NPs in various media. This is extremely important for practical uses, not only for those mentioned above but also for building and construction applications, particularly in tropical countries where solar energy is abundant.
There are several approaches for preparing SiO2-coated metal NPs and metal oxide NPs. One example is the Stöber method, which involves the base-catalyzed hydrolysis of a silane precursor such as tetraethyl orthosilicate (TEOS) followed by the formation of SiO2 on the surface of metal NPs and metal oxide NPs. Although this method is very useful, the rate of production of the SiO2-coated metal NPs and metal oxide NPs using this method is very slow due to the inertness of the TEOS, leading to difficulty in large-scale production.
Further, conventional production of SiO2-coated metal NPs and metal oxide NPs requires the use of at least one organic solvent. For example, in the Stöber method, ethanol is used. The organic solvent usually comprises greater than 80% of the solvent mixture. This is because the silane precursors used in conventional methods are not soluble in water. As such, addition of organic solvent such as ethanol is necessary to ensure that the silane precursor dissolves in the reaction mixture. However, the requirement to add an organic solvent to the reaction mixture leads to additional steps in the preparation and purification of the NPs. Further, the use of organic solvents makes the process less environmentally friendly.
Further, conventional approaches often also require the treatment of the surface of the metal or metal oxide NP by surface-stabilizing agents or surface-modifying agents prior to SiO2 coating. That is, the SiO2 is coated on the surface indirectly. This leads to multiple steps being involved in the preparation, resulting in a complicated synthetic procedure.
Another approach for synthesizing SiO2-coated metal NPs and metal oxide NPs may be to use reverse microemulsion, whereby surfactants are used for making the water-in-oil emulsion. However, this leads to additional purification steps to remove the surfactant.
The use of organic solvents, surface-stabilizing agents, surface-modifying agents and surfactants makes these approaches inefficient, time-consuming, environmentally unfriendly, very costly, and unsuitable for large-scale production.
There is therefore a need to provide a method for preparing SiO2-coated metal NPs and metal oxide NPs that overcomes or at least ameliorates, one or more of the disadvantages described above.